Hi @CAMatthew, thanks for posting on the forum and welcome to the SimScale Community 
For us to take a closer look at your model, please share a link to the project you’re working on. Here’s an explanation on how to do that: Guide to Clear and Understandable Forum Posting
When these sorts of issues happen, though, the best approach is usually to first try and export the CAD in another format and perhaps from another tool (have you tried OnShape already?). Translation problems might occur and changing the generation process can be helpful.
We also have this documentation which lays out some of the most common CAD faults:
Cheers
Igor